Linking health facility data from young adults aged 18-24 years to longitudinal demographic data: Experience from The Kilifi Health and Demographic Surveillance System by Nyundo, C, Doyle, A, Walumbe, D, Otiende, M, Kinuthia, M, Amadi, D, Jibendi, B, Mochamah, G, Kihuha, N, Williams, TN, Ross, D, Bauni, E